Plasma processing apparatuses are used in these days in the fabrication process of semiconductor devices, in view of increase of integration density and increased degree of device miniaturization, for film formation, etching, ashing, and the like. Particularly, a microwave plasma processing apparatus that induces plasma by using microwave has an advantage of generating stable plasma even in a high vacuum state of relatively low pressure, such as 0.1—several 10 mTorrs. Because of this, a microwave plasma processing apparatus that uses a microwave such as 2.45 GHz now draws attention.
In the microwave plasma-processing apparatus, there is provided a dielectric plate transparent to microwave on a ceiling part of a processing vessel, while the processing vessel is connected to a general evacuation system. Further, a flat, disk-like antenna member (microwave-emitting member) is provided on the dielectric plate.
Further, a retardation plate is provided on the antenna member. The retardation plate is formed of a dielectric film having a predetermined dielectric constant and compresses the wavelength of the microwave supplied thereto from a microwave generator with a predetermined ratio. The antenna member is formed with a number of penetrating holes (slots) and the microwave supplied to the central part of the antenna member is introduced into the processing vessel through the slots after propagating inside the antenna member in the radial direction thereof. As a result of the compression of the wavelength of the microwave supplied to the antenna member by the retardation plate, the size of the slots formed in the antenna member is reduced, and as a result, the uniformity of plasma density is improved by providing a large number of slots on the antenna member.
In the microwave plasma processing apparatus of such a construction, the microwave introduced into the processing vessel from the antenna member via the dielectric place induces plasma of a processing gas, and the semiconductor wafer disposed in the processing vessel is subjected to plasma processing.
The microwave introduced into the processing vessel via the retardation plate and the antenna member is supplied from a microwave generator such as a magnetron, or the like. The microwave generated by the microwave generator is supplied to the retardation plate or to the antenna member via a waveguide. The microwave thus guided through the waveguide is emitted as it is propagated through the antenna member in the radial direction thereof from the central part. Ultimately, the microwave is emitted uniformly into a process space inside the processing vessel.
A coaxial waveguide has been used generally for supplying the microwave to the central part of the retardation plate or the antenna member. Thus, the microwave generated by the microwave generator is supplied to a waveguide having a rectangular cross section and is propagated therethrough to the part generally at the center of the retardation plate or the antenna member. There, the microwave is supplied into the central part of the retardation plate or the antenna member via the coaxial waveguide formed of an inner conductor and an outer conductor.
The coaxial waveguide that supplies the microwave to the central part of the retardation plate or the antenna member is formed for example of an inner conductor and an outer conductor, wherein the inner conductor is formed of a thin tube having a diameter of about 17 mm.
Thus, in the case large electric power is to be supplied to the antenna member via the coaxial waveguide, there is induced a very strong electric field in the vicinity of the inner conductor of the coaxial waveguide, and the temperature of the inner conductor is raised as a result of heating. There, a part of the microwave energy is converted to heat, while such conversion of microwave energy to heat increases the loss (conductor loss) of the supplied microwave. Thereby, the efficiency of electric power feeding is deteriorated. Further, there arises the need of cooling the coaxial waveguide by covering the same by a cooling device, while such a construction increases the size and cost of the apparatus.
Further, because of the existence of very strong electric field in the vicinity of the inner conductor, there is a possibility that abnormal electric discharge may be caused in the vicinity of the inner conductor. When such abnormal electric discharge is caused, the retardation plate may be damaged in the part thereof near the inner conductor.
For example, there is a need of connecting the inner conductor of the coaxial waveguide to the antenna member in the state that the coaxial waveguide makes an intimate contact with the central part of the antenna member, and because of this, it is practiced to connect the inner conductor to the antenna member by screwing a screw into a threaded hole provided inside the inner conductor through the retardation plate in the state the end surface makes a contact with the antenna member. With such a connection that uses a screw, however, it is difficult to achieve complete and intimate contact of the inner conductor end surface to the surface of the antenna member. When there is a minute gap, on the other hand, there tends to occur abnormal electric discharge in such a part.
Further, in the screwed connection explained above, there exists a screw head at the junction part where the dielectric plate and the antenna member (slot antenna) are joined. Thus, there is a need of forming a depression in the dielectric plate at the central part thereof for accommodating therein the screw head projecting from the surface of the slot antenna. Thus, it is difficult to achieve an intimate connection between the retardation plate and the antenna member at the central part, and there arises the problem that abnormal electric discharge takes place in the vicinity of such a depression.
Further, there can be a case in which it is desired to confine a helium (He) gas between the antenna member or the retardation plate and the top plate formed of a dielectric material, for facilitating thermal conduction. In such a case, there is provided a seal ring at the outer periphery of the inner conductor constituting the coaxial waveguide for achieving sealing with respect to the dielectric top plate. In such a case, however, there arises a problem in that the seal ring may be damaged as a result of degradation caused by the exposure to the strong microwave.
Thus, there is a proposal to provide a cavity resonator between the waveguide and the antenna member and introduce the microwave into the processing vessel from the cavity resonator. The Patent Japanese Patent 2,569,019 discloses the technology of providing such a cavity resonator in the waveguide for amplifying the microwave. In this technology, the microwave thus amplified is introduced into the processing vessel through a number of slots formed on the bottom surface of the cavity resonator. Thus, the cavity resonator disclosed in this patent publication is formed with a number of slots on the entire bottom surface thereof so that there occurs emission of the microwave from the entire bottom surface facing the processing vessel. Therefore, the foregoing prior art does not have the construction of causing radial propagation of the microwave from the central part when emitting the microwave into the processing space of the processing vessel. Further, it should be noted that the length of each slot is determined by the wavelength of the microwave. In the case of using the microwave of 2.45 GHz, the slot desirably has the length of 60 mm or more in correspondence to the half wavelength of the microwave. This means that the construction of the reference does not provide the desirable effect of the microwave propagated in the radial direction from the central part is emitted uniformly into the processing vessel from a number of minute slots. Thus, it is not possible to apply the microwave supplying mechanism (feeding mechanism) having a cavity resonator as is disclosed in the foregoing Patent Japanese Patent 2,569,019 to the antenna member (slot antenna) that supplied the microwave uniformly to the processing vessel from a number of minute slots.